1. Technical Field
The disclosure relates to a circuit that is capable of reducing electromagnetic interference, and more particularly, to a control circuit for reducing electromagnetic interference.
2. Related Art
With advances in digital technology, in order to be able to support the high-density and high-speed information transmission, a reliable and accurate clock generator is required in a digital system to generate a high-frequency clock signal serving as a reference to the transmission rate of data signal. A conventional system clock is generated by a series of functional elements such as chipsets or independently packaged highly integrated IC. However, the ICs are susceptible to the electromagnetic interferences (EMI).
The aforementioned EMI caused by the high-frequency clock signal generated by a clock generator has a negative impact on the reliability and the performance of the digital system. For minimizing such an impact arising out of signal switching at high frequencies, an EMI filter or a snubbing device is disposed at a power input end. However, these additional components increase the manufacturing cost of the entire circuitry.
In order to reduce the energy caused by the high frequency clock signals to reduce the corresponding electromagnetic interference, U.S. Pat. No. 6,249,876 employs a logic manner to output control signals for determining an output current of a digital-to-analog conversion device (D/A converter), and then combines the output current with the current in the oscillator so as to set the frequency of signal variously. However, the current used in the oscillator is usually several micro-amperes (μA), but the output current of the D/A converter is required to be less than several tens nano-amperes (nA). As a result, the variation in the currents may not be properly controlled, and thus the frequency may not be controlled in a desired fashion.
In addition, U.S. Pat. No. 7,289,582 utilizes an oscillator to output an input reference voltage to a voltage control unit, and by changing the input reference voltage, changes the voltage of an output clock of the oscillator, so as to reduce the electromagnetic interference. However, many resistors connected in series are required to form the multiple input reference voltages (e.g., 2n input reference voltages), and many switches are required to select the output voltage of the voltage control unit. This causes the entire circuit design complicated.